Question: Multiply.
Explanation: When a number is next to a variable, it means they are multiplied. So, $({8}{d^4})({-8}{d^4})$ is the same as $({8})({d^4})({-8})({d^4})$. Now we can rearrange the factors because multiplication is commutative (a fancy way of saying that the order in which we multiply things doesn't matter). ${(8)(-8)}{(d^4)(d^4)}$ Then simplify, and we're done! ${-64}{d^8}$